1. Field of the Invention
The present invention relates to a pickup device suitable for a video disc player of a type in which a scanning needle scans a record track while it is in contact with a disc.
2. Description of the Prior Art
It is well known that a video disc system has two major types, one being a contact type video disc such as a piezoelectic system or an electrostatic capacitance system in which a scanning needle scans a record track while it is in contact with a disc, and the other being a non-contact type video disc such as an optical system in which the scanning needle scans the record track while it is not in contact with the disc. Each of those types has advantages and disadvantages. Among others, the contact type video disc has a big disadvantage that the disc and the scanning needles are damaged or worn because the scanning needle contacts the disc.
Another big disadvantage of the contact type video disc is that tracking is unstable as the contact condition of the disc and the scanning needle changes. In this type of video disc, it has been proposed to form a guide groove to guide the scanning groove and form a record track in the guide groove to permit tracking of the scanning needle, or, in the electrostatic capacitance system, to form no guide groove but use a pilot signal to permit tracking of the scanning needle. In these systems, the tracking is stable when the scanning needle contacts the disc with an appropriate needle pressure. However, when the contact condition of the scanning needle and the disc changes by surface vibration of the disc resulting in the decrease of the needle pressure, the scanning needle is ready to move away from the disc and the tracking is unstable.
In a prior art pickup device in which the needle pressure is applied by a resilient member which supports a cantilever on which the scanning needle is mounted, a displacement characteristic of the resilient member is appropriately selected such that the decrease of the needle pressure is small even when the surface vibration of the disc is included so that the scanning needle can track the surface vibration.
The disc includes a wow, a flutter and an excentricity which cause jitter in a reproduce video signal. Accordingly, the prior art pickup device has jitter compensation means for mechanically vibrating the scanning needle in a direction of the record track in accordance with the detected jitter. However, when the scanning needle is vibrated in this manner, a displacement of the scanning needle by the vibration causes a change of the contact condition of the scanning needle and the disc. This is discussed in further detail.
FIG. 1 shows a side view of a prior art pickup device and
FIG. 2 shows a side view illustrating a displacement of a scanning needle when it is compensated for jitter.
Numeral 1 denotes a cantilever, numeral 2 denotes a tip holder, numeral 3 denotes a scanning needle, numeral 4 denotes a fly reed, numeral 5 denotes a coupling rubber, numeral 6 denotes a permanent magnet, numeral 7 denotes a support, numeral 8 denotes a support rubber, numeral 9 denotes a jitter compensation driver, numeral 10 denotes a drive coil, numeral 11 denotes a disc and numeral 12 denotes a disc surface.
The operation of the prior art device is explained below.
Referring to FIG. 1, the tip holder 2 having the scanning needle 3 fixed thereto is mounted at one end of the cantilever 1, which in turn is supported by the support 7 through the coupling rubber 5. The support 7 is supported by the support rubber 8 and the permanent magnet 6 is fixed to one end thereof. The drive coil 10 of the jitter compensation driver 9 is arranged around the permanent magnet 6. When a player is in a rest position, the cantilever 1 is lifted by a cantilever lifter, not shown so that the scanning needle 3 is off the disc surface 12. In a play position, the cantilever 1 is descended so that the scanning needle 3 is in contact with the disc surface 12 as shown. The resilient fly reed 4 applies an appropriate needle pressure to the scanning needle 3 to bring the scanning needle 3 into contact with the disc surface 12. The fly reed 4 is made of conductive material and connected to an electrode (not shown) of the scanning needle 3. The fly reed 4 serves as a transmission line to supply a signal picked up at the electrode to a reproducing signal processing circuit, not shown. When a signal representing the amount of jitter detected by the reproducing signal processing circuit is supplied to the driver 9, a jitter compensation drive current flows through the drive coil 10 which generates a magnetic field which in turn acts an the permanent magnet 6 to create a drive force to displace the cantilever 1 in the direction of an arrow A or B. As a result, the scanning needle 3 is restricted to the disc surface 12 and displaced in the direction of an arrow C or D, that is, in the direction of the record track. In this manner, the jitter is eliminated.
Since the pickup device is constructed as described above, the cantilever 1 is inclined to the disc surface 12 by an angle .theta. when the cantilever 1 descends to the play position. Thus, as the cantilever 1 is displaced in the direction of the arrow A or B, a force parallel to the cantilever 1 is applied to the scanning needle 3 so that the scanning needle 3 is displaced by the disc surface 12 in the direction parallel thereto (the direction of the arrow C or D), and a force normal to the disc surface 12 also acts to the scanning needle 3. Accordingly, when the cantilever 1 is displaced in the direction of the arrow A, an urging force toward the disc surface 12 is applied to the scanning needle 3 so that the needle pressure is increased. As a result, the disc 11 or the scanning needle 3 may be damaged. On the other hand, when the cantilever 1 is displaced in the direction of the arrow B, a separation force away from the disc surface 12 acts to on the scanning needle 3 so that the needle pressure decreases and the tracking is unstable.
The direction of the drive force created by the mutual action of the magnetic field by the drive coil 10 and the permanent magnet 6 is in the longitudinal direction of the cantilever 1. As shwon in FIG. 2, if a line connecting a support point P of the support 7 and a center Q of the permanent magnet 6 does not coincide with the drive force F, a torque is created in the support 7 around the support point P so that the support 7 is rotated. As a result, the cantilever 1 is rotated in the direction of an arrow K. If the rotational direction is normal to the disc surface 12, a force to change the needle pressure is applied to the scanning needle 3. As a result, the scanning needle 3 or the disc 11 may be damaged and the tracking is unstable. Since it is very difficult to construct the device such that the line connecting the support point P and the center Q of the permanent magnet 6 exactly coincides with the drive force F, the above problem cannot be resolved in the prior art device.
As described above, the prior art pickup device cannot avoid the damage of the disc and the scanning needle and the increase of the wear due to the action of the jitter compensation means and results in instability of the tracking.